Alternaria alternata f. sp. lycopersici
Symptoms may occur on all above ground parts of the plant. Dark brown
cankers with concentric rings form on stems and are often associated
with wound sites. These cankers may enlarge, eventually girdling the
stem and killing the plant. A brown, dry rot develops and brown streaks
can extend into the pith above and below the cankers. A toxin produced
by the fungus growing in the stem canker moves into the upper part of
the plant, killing the interveinal leaf tissue. As the disease
progresses, curling of leaf margins and eventual death of the leaf
occurs. Fruit symptoms initially appear as small, gray flecks which
later enlarge and become dark and sunken with the characteristic
concentric rings. Symptoms may not be present on mature green fruit,
however, they can develop rapidly in transit.
Conditions for Disease Development:
The fungus can survive in the soil and crop debris for more than a year.
Rain, overhead irrigation and dews favor development of this disease,
and its conidia are easily spread by the wind. Stem wounds created by
pruning allow entry of the fungus, however, infection can also occur
It is difficult to control this disease with fungicides so resistant
varieties should be used.
black stem lesions.
Close-up of the stem canker.
induced leaf interveinal
sunken fruit lesions.